Polyacetal resins have been used in various applications such as electric equipment and parts thereof, automobile parts and other mechanical parts, because they are excellent in balance of mechanical strength, chemical resistance and slidability and also in workability.
Recently, a demand for a polyacetal resin article being compact, thin-walled or precise has been increasing. As a result, molding methods or conditions wherein higher thermal history is applied are more often employed for molding. For example, molding using a pin-gate type mold, high-cycle molding, or a molding method for a compact, thin-walled or precise part using a polyacetal resin having higher viscosity can be enumerated. In these methods, higher thermal history is given to a resin compared to usual molding methods by raising a shear rate or by increasing screw rotation speed or molding temperature for reducing a plasticizing time. Even when a usual molding method is employed, higher thermal history is caused. This is because a molding temperature is often increased to avoid molding defects such as flow mark, weldline, and jetting. Moreover, in the case in which a hot-runner type mold is used, resin decomposition may occur owing to increase of a resin temperature caused by partial accumulation of resin.
As the resin temperature goes up in a molding machine, polyacetal resin becomes more vulnerable to heat decomposition, self oxidation decomposition, decomposition by acid or base, decomposition by residual active catalyst or the like, and more formaldehyde is disadvantageously generated owing to the decomposition. The conventional polyacetal resins are not suitable for the molding method wherein higher thermal history is applied.
In the field of automobile interior parts, it has been increasingly demanded to reduce an emission amount of volatile organic compounds (VOC) such as formaldehyde. As described above, polyacetal resin has the feature of generating formaldehyde by decomposition of main chain. However, proposals satisfying the above demand have not yet been made.
Various methods have been suggested to reduce an amount of formaldehyde emitted from an article of a polyacetal resin. For example, there have been suggested to add a polyamide and a hydrazine derivative (Patent Document 1); to add a hydrazide compound (Patent Document 2); to add a nitrogen compound selected from a melamine, a melamine derivative and di-carboxylic acid di-hydrazide (Patent Document 3); to add a benzoguanamine (Patent Document 4); to attach a partial aliphatic acid ester of a polyhydric alcohol to the surface of pellet (Patent Document 5); to add a mono N-substituted urea (Patent Document 6); to add a compound including carboxyl group having acid dissociation index of not smaller than 3.6 (Patent Document 7); to add a condensate of phenols, a compound including basic nitrogen and aldehydes (Patent Document 8); to add hydantoin derivatives or imidazole derivatives (Patent Document 9); to add a low molecular weight amino compound having a basic dissociation index of 2 to 8 (Patent Document 10); to add a spiro compound having cyclic triazine (Patent Document 11); and the like.
Patent Document 1: JP-A-51-111857
Patent Document 2: JP-A-4-345648
Patent Document 3: U.S. Pat. No. 5,212,222 (JP-A-7-173369)
Patent Document 4: EP 235 927 A2 (JP-A-62-190248)
Patent Document 5: WO95/16734 (JP-A-6-107900)
Patent Document 6: JP-A-11-335519
Patent Document 7: JP-A-2000-239484
Patent Document 8: JP-A-2002-212384
Patent Document 9: U.S. Pat. No. 5,866,671 (JP-A-8-41288)
Patent Document 10: WO00/59993
Patent Document 11: U.S. Pat. No. 6,673,405
The present inventors have found that among these suggested compounds, a hydrazide compound is most effective to reduce an amount of formaldehyde emitted from an article. According to Patent Document 2, however, the hydrazide compound is not effective to inhibit generation of formaldehyde caused by decomposition of a polyacetal resin in molten state. Therefore, the hydrazide compound is not suitable for molding as described above or molding causing partial rising of resin temperature. Moreover, according to Patent Document 9, the use of a hydrazide compound might be limited depending on application. That is, a hydrazine might be generated by hydrolysis of a hydrazide compound or the like under a certain condition so that the use of the hydrazide compound is sometimes limited depending on with which additives the compound is combined. Furthermore, addition of the hydrazide compound in a large quantity might cause mold deposit during molding or bleed-out to the surface of articles. As described above, the hydrazide compound has the problem that the use thereof is limited, though superior in inhibition of formaldehyde generation.